U.S. Pat. No. 5,357,802 (Hofmeyer) describes how a gravity gradiometer may be constructed with plural accelerometers carried by a rotating disc to find changes in the magnitude of the gravitational field.
Hofmeyer also describes how using a matched pair of accelerometers mounted on opposite ends of a common diameter of the disc facilitates comparison of the signals produced. In addition this arrangement provides for cancellation of any displacement of the gradiometer. Such an instrument is said to employ a disc rotation rate of about 15 rpm (revolutions per minute).
Hofmeyer then goes on to describe a problem which arises when it is desirable to increase the rotational rate of the disc, say up to 50 or, 100 rpm. This problem arises when the mass to be sensed moves past the gravity gradiometer (or vice versa) in a time which is short compared with the rotation period of the gravity gradiometer. It exhibits itself as noise in the form of additional signals which are a function of the rotational speed of the disc.
Hofmeyer addresses this problem by proposing the use of a set of four accelerometers mounted on a rotating disc. The four accelerometers are arranged in two opposed pairs mounted at the ends of perpendicular diameters. Signal processing is accomplished by adding signals of diametrically opposed accelerometers to obtain sum signals, then taking the difference between the sum signals and demodulating the difference at twice the rotational frequency. The output signal includes a higher order frequency term as a function of rotational frequency. This rotational frequency term can be cancelled by subtraction from another signal obtained from another identical set of four accelerometers.
The problem dealt with by Hofmeyer is not generally an issue in airborne surveys for minerals. Here the spatial extent of a deposit, and the spreading of the gravity gradient effect of the deposit due to the separation of the deposit from the aircraft, mean that the extent of the anomaly is generally of the order of 400 m and at common survey speeds, such as 70 m/s, this is greater than the period of rotation.